Purl Stitch
Purl stitch is the other knitting stitch that you'll need to know to knit my birds, and many other knitting patterns.
It's used in combination with knit stitch to create all sorts of different textures and patterns, but the main one that you will find in the knitted birds patterns, is stocking stitch.
Stocking stitch is when you work one row in knit stitch, all the way to the end of the row, and then you work in purl stitch, all the way to the end of the row, and then keep repeating this pattern of one row knit, one row purl. Beneduck Cumberquack's wings are worked in stocking stitch. It looks like a series of little "v"s.
Here's an excellent purl stitch tutorial from Love Crafts, which should have you up and running with purl stitch in no time at all!